LOWELL, Mass. — When Republican Sen. Scott Brown proposed a pledge to eliminate third-party ads in his race with Democrat Elizabeth Warren, it had all the makings of a well-crafted but wonkish piece of political theater. Press reviews would be positive, but its run would be short.

Surprisingly, 10 months and tens of millions of dollars later, the simple contract — imposing steep penalties on the candidates if the pledge was broken — has held. There have been many negative ads in this white-hot contest that could determine control of the chamber, but each begins or ends with the candidate standing by the spot.

The victor of the Senate race will be able to plausibly claim that the agreement — with its alliterative royal moniker, the People’s Pledge — was to his or her benefit. What is certain is that it irrevocably changed the contours of one of the most hard-fought races in the country.

How it did remains a matter of partisan perspective.

“Whoever wins will look back and say, ‘This helped,’” veteran Massachusetts Democratic strategist Mary Anne Marsh said. She explained that both candidates benefited in different ways.

Marsh said the pledge gave Warren, especially early in the campaign, “some breathing room for people to get to know her on her own terms without third parties coming in here and defining her negatively.”

This year, Warren, a Harvard University professor and consumer advocate, was able to spend millions of dollars on positive biographical spots about herself before anyone disputed that narrative in paid TV ads.

But when the pledge was inked in early January, it was immediately advantageous to Brown. Before the agreement was signed, Brown was getting hammered by outside groups that were eroding his likability. Marsh, along with other Massachusetts Democratic and Republican strategists, said the cessation of those spots helped him stanch the bleeding.

“He was getting buried by these negative ads,” a Brown campaign aide said, pointing to polling that showed the negative ads, particularly from the League of Conservation Voters, had swayed public opinion against the Senator. “If that had continued, his image would have been ground to a pulp with all the outside spending.

“It’s hard enough to win as a Republican in Massachusetts. It’s impossible to win as a disliked Republican here,” the aide added. “Scott Brown would have been left for dead if those ads had kept running.”

Democrats insist, however, that Brown’s likability has taken a hit from having to stand by his attack ads on Warren. Without third-party groups to do much of the heavy negative lifting for him, he has had to risk his nice-guy image by saying he approves messages savaging Warren’s narrative.